1. Field of the Invention
The present invention relates to an instrument current transformer for power cables and, in particular, relates to an instrument current transformer for power cables which is provided so as to surround power cables connected to a gas insulated switch gear device.
2. Description of Related Art
An instrument current transformer which is provided for a gas insulated switch gear device is usually disposed in a container filled with an insulating gas such as in a container accommodating cable heads, however, in accordance with a change in view point on a protection range in a power transmission system with an instrument current transformer due to an improvement in a reliability of electrical equipments, an instrument current transformer for power cables which is provided so as to surround the power cables connected to a gas insulated switch gear device has become to be used.
Examples of the above instrument current transformers for power cables are disclosed in JP(U)-A-59-179418(1984) and JP(U)-A-61-14819(1986). In the former, the power cables are disposed beneath the floor on which a gas insulated switch gear device is placed and the instrument current transformer for the power cables is constituted by current transformer coils surrounding these power cables. Further in the latter, the instrument current transformer for power cables is constituted by current transformer coils surrounding power cables disposed at a position above the floor on which the gas insulated switch gear device is placed, and between the installation floor and the lower end of a closed container which accommodates cable heads constituting the connecting member for the power cables.
However, because of a shortage of space in urban areas, current gas insulated switch gear devices are frequently built in underground spaces of buildings as underground substations so that a further small sizing and lowering stacking height of the gas insulated switch gear device are required. Therefore, with regard to the former conventional instrument current transformer for power cables, when there is an under story below the floor on which the gas insulated switch gear device is placed, the installation work of the instrument current transformer for power cables is easy. However when the gas insulated switch gear device is built in an underground space with no under stories, the work of laying power cables in an under hole and combining the current transformer coils therewith become very hard. Further, with regard to the latter conventional instrument current transformer for power cables, the distance between the installation floor and the lower end of the closed container accommodating the cable heads is more and more reduced because of the small sizing and stacking height lowering of the gas insulated switch gear device, and it becomes difficult to obtain a space required for the disposition and the installation work.